We investigate several preliminary models for the muscle stress produced during the swimming of a hydromedusa, Sarsia tubulosa, which relate the pressure on the hydromedusa to the stress in the musculature. Our calculations are based on the solution of the Navier-Stokes equations for the flow around a swimming hydromedusa. This numerical model allows us to obtain detailed pressure information in any area of interest on or near the hydromedusa. Initially, we investigate the pressure distribution on the surface of the hydromedusa and find that it is quite uniform in space. We present an original model based on a force balance created using the actual hydromedusa’s geometry and compare the results we obtain to the stress in a pressurized thin walled cylinder or sphere of equal radius. Our model allows us to calculate and estimated muscle stress at any point along the hydromedusa. By doing so, we are able to validate the assumptions made by modeling the subumbrellar cavity as a pressurized cylinder or sphere.

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